Twenty years ago, physicists first saw a hint of the existence of an exotic type of matter made up of four neutrons. Now, they appear to have observed it in the lab for the first time. 

Neutrons and protons make up the nucleus of almost all elements in the universe, sticking together thanks to the strong nuclear force. Protons don’t stick together because their electromagnetic charge being positive pushes them apart. Neutrons have no charge but keeping them confined is also a challenge.

Neutron stars, extreme objects with incredible density are capable to keep the neutrons stuck together due to their gravity. But physicists have long sought to create a particle that is made by just four neutrons without extreme physics. And, as reported in Nature, they have now achieved this long-sought quest.

“This experimental breakthrough provides a benchmark to test the nuclear force with a pure system made of neutrons only," lead author Dr. Meytal Duer from the Institute for Nuclear Physics at the TU Darmstadt, said in a statement. 

“The nuclear interaction among more than two neutrons could not be tested so far, and theoretical predictions yield a wide scatter concerning the energy and width of a possible tetra neutron state

The team was able to achieve it by shooting an isotope of helium, called helium-8 – which has four extra neutrons compared to the most common version – to a target made of liquid hydrogen. The interaction results in the hydrogen being kicked, the release of a helium-4 atom, and the remaining 4 neutrons can simply interact among themselves.

The state is called a tetra neutron (tetra is Greek for four) and the production of this peculiar particle can allow for new studies on the property of neutrons but also of neutron stars. The technique used was different from what has been employed previously to create such a particular interaction.

“Key for the successful observation of the Tetra Neutron was the chosen reaction, which isolates the four neutrons in a fast (compared to the nuclear scale) process, and the chosen kinematics of large momentum-transfer, which separates the neutrons from the charged particles in momentum space,” said Professor Dr Thomas Aumann, also from the Institute for Nuclear Physics. 

“The extreme kinematics resulted in an almost background-free measurement. We now plan to employ the same reaction in an experiment at the RIBF to make a precision measurement of the low-energy neutron-neutron interaction. A dedicated neutron detector for this experiment is currently being built at our university”.